1. Field of the Invention
The invention generally relates to multilayer polyolefin film structures and methods of making the same. More specifically, the invention relates to a biaxially-oriented polypropylene film incorporating a non-crystallizable, amorphous polyester layer.
2. Brief Description of Related Technology
Biaxially-oriented polypropylene (BOPP) films are widely used in packaging because they have good stiffness, strength, optical properties (low haze and high gloss), and moisture barrier properties. Users of packaging films, particularly users of BOPP films, are continually seeking structures with improved printability, metallizing properties, and gas barrier. Because of their olefinic nature, typical BOPP constructions have low surface energy and require treatment (corona, flame, etc.) in order to be printed or metallized. Polyester is known to have high surface energy and possesses excellent printing and metallizing attributes. Additionally, polyester, both in clear and metallized structures, offers improved gas barrier performance as compared to BOPP films. This is especially true in the case of metallized polyester films which are an order of magnitude or more lower in oxygen transmission rate.
Although there has been a long felt need for BOPP films incorporating polyester layers, problems in obtaining acceptable optical and processability characteristics, adequate interply adhesion, and other properties have been encountered in previous attempts to produce polyester-containing BOPP structures. For example, U.S. Pat. No. 5,324,467 discloses a process for the preparation of an oriented multilayer laminate film having at least three layers, including a polypropylene layer, a tie layer, and a copolyester layer. The films are formed by combining the layers in the molten state, either in a coextrusion, or in separate extrusions brought together outside the die (where the film is subsequently cooled, oriented uniaxially or biaxially, and then heat set to lock in the properties). A major problem in producing a structure according to this method on commercial scale equipment is the strong tendency of polyester to adhere to the heated metal rolls of the machine direction orientation section. This makes it difficult to achieve good optical properties free of visual defects and may also decrease other properties such as the seal initiation temperature.
U.S. Pat. No. 4,874,656 describes a multilayer laminate having a high mechanical resistance and an impermeability to gases and vapors. In the disclosed structures, a polyester layer is joined to a polypropylene layer after the polypropylene is biaxially-oriented, the polyester layer is quite thick (12 to 24 microns), and the structure includes a layer of metallic foil and a layer of polyethylene. While joining a polyester layer to a BOPP layer after biaxial orientation is possible, this method is impractical for incorporating thin layers of polyester.
U.S. Pat. No. 4,924,525 also describes a structure wherein a polyester laminate is adhered to a BOPP film after the polypropylene is biaxially oriented. Again, this invention precludes the use of thin polyester layers in the final structure.
U.S. Pat. No. 5,637,366 describes a structure wherein a three-layer polyester-containing cap layer is bonded to a monoaxially oriented polypropylene core, followed by orientation of the resulting composite in a direction transverse to the direction of orientation of the core layer. The '366 patent however teaches that the polyester layer utilizes as a major portion thereof (e.g., >89%), crystalline polyesters, crystallizable amorphous polyester homopolymers, and crystallizable amorphous polyesters, an example of which is described in the '366 patent as a polyethylene terephthalate isophthalate sold by E.I. du Pont de Nemours & Co., Wilmington, Del., under the name SELAR® PT8307. The structure disclosed in the '366 patent can experience seal strength problems in that the cap layer to core seal strength may not be consistently high enough for use of the film in certain applications.